RJ Metrics selected 40,000 Google+ users at random. It then analyzed their public posts.

What they found is that a lot of people start sharing on Google+, then stop. 3 out of 10 made a single public post, then never posted again. Even among people who made five posts, 15 percent had stopped posting.

RJ Metrics said this "decay rate" was disturbing.

Other analysts have found that people spend an average of 3 minutes a month on Google+, versus 7 hours on Facebook.

Now, it's possible that many Google+ users are not posting publicly and are sharing privately instead, as Google+ allows. That's Google's timeworn excuse when asked about Google+ engagement. But Google has refused to give clear statistics about activity on Google+.

"Google is just refusing to answer the question for its own reasons, which is probably because Google+ has far less activity as a standalone social network than either Facebook or Twitter," wrote Google expert Danny Sullivan recently.